I believe one must be logged on with an administrator account or an account with administrator privileges for one to be able to install in the first place. If you make only one separate Limited User
account for everyone to use, you will make sure no one will be able to install anything. Not only will they not be able to install anything, they'll also not be able to play around with system settings/wreck the computer. So, I would recommend this option instead of looking for a program which prevents installation without password.
Edited by Juha, 07 July 2008 - 11:15 PM.